Cash Equivalents and Investments - Unrealized Loss Position on Fixed Income Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fixed Income Securities
Fair Value
Less than 12 Months	$ 989.4	$ 1,238.3
12 Months or Greater	292.8	273.3
Total	1,282.2	1,511.6
Unrealized Loss
Less than 12 Months	(2.8)	(3.6)
12 Months or Greater	(2.6)	(1.8)
Total	(5.4)	(5.4)
Asset-backed securities
Fair Value
Less than 12 Months	16.9	215.2
12 Months or Greater	46.5	38.4
Total	63.4	253.6
Unrealized Loss
Less than 12 Months	(0.1)	(0.4)
12 Months or Greater	(0.3)	(0.2)
Total	(0.4)	(0.6)
Commercial paper
Fair Value
Less than 12 Months	0.8
12 Months or Greater	0.0
Total	0.8
Unrealized Loss
Less than 12 Months	0.0
12 Months or Greater	0.0
Total	0.0
Corporate debt securities
Fair Value
Less than 12 Months	279.0	646.7
12 Months or Greater	136.9	108.6
Total	415.9	755.3
Unrealized Loss
Less than 12 Months	(2.3)	(2.1)
12 Months or Greater	(1.4)	(0.9)
Total	(3.7)	(3.0)
Foreign government debt securities
Fair Value
Less than 12 Months	8.0	47.3
12 Months or Greater	13.4	6.6
Total	21.4	53.9
Unrealized Loss
Less than 12 Months	0.0	(0.2)
12 Months or Greater	(0.2)	0.0
Total	(0.2)	(0.2)
U.S. government agency securities
Fair Value
Less than 12 Months	67.8	68.3
12 Months or Greater	33.6	67.9
Total	101.4	136.2
Unrealized Loss
Less than 12 Months	(0.1)	(0.2)
12 Months or Greater	(0.2)	(0.5)
Total	(0.3)	(0.7)
U.S. government securities
Fair Value
Less than 12 Months	616.9	260.8
12 Months or Greater	62.4	51.8
Total	679.3	312.6
Unrealized Loss
Less than 12 Months	(0.3)	(0.7)
12 Months or Greater	(0.5)	(0.2)
Total	$ (0.8)	$ (0.9)